This invention relates to an adsorbent for removing HF and related organic fluorides from fluid streams in which they are contained as impurities, in particular, from hydrocarbon fluid streams in petroleum refineries. This invention further relates to a method of using such adsorbent to remove HF and related organic fluoride compounds from fluid streams in which they are contained as impurities and, in particular, from hydrocarbon streams downstream from acid catalyzed alkylation processes.
The alkylation reaction as practiced in petroleum refining involves the condensation of an olefin (ethylene, propylene, butylenes, and amylenes) with isobutane to yield high-octane branched-chain hydrocarbons in the gasoline boiling range. Olefin feedstock for the alkylation reaction is typically that which is derived from the gas make of a catalytic cracker. Isobutane is usually obtained from refinery gas recovery or it is produced by catalytic butane isomerization. Alkylation can be accomplished as a thermal, thermal-catalytic, or catalytic reaction. Hydrofluoric Acid Alkylation is a catalytic reaction in which hydrofluoric acid (HF) is used as the catalyst.
As a result of the use of the hydrofluoric acid catalyst, HF Alkylation Unit effluent streams inevitably contain trace levels, that can be up to several hundred parts per million by weight, of fluoride-containing compounds, namely free hydrogen fluoride, related organic, or combined, fluorides, and mixtures thereof. These species are considered to be impurities or contaminants and must be removed in order to avoid corrosive effects and/or meet product specifications.
As used herein, the term xe2x80x9crelated organic fluoridesxe2x80x9d refers to those compounds that are formed in the HF Alkylation reactor by the addition of hydrofluoric acid across an olefinic double bond:
R2Cxe2x95x90CR2+HFxe2x86x92R2HCxe2x80x94CFR2
The term xe2x80x9ccombined fluoridesxe2x80x9d is synonymous with organic fluorides, thus referring chemical species resulting from the reaction, or combination, of HF with an unsaturated, or olefinic, organic compound. The term xe2x80x9ctotal fluoridesxe2x80x9d refers to the summation of free HF and combined fluorides.
Both free HF and related organic fluorides present in the reactor effluent will throw salable LPG product off fluoride specifications as well as pose corrosion and environment concerns. One or more of the products from an HF Alkylation Unit operation may be treated for fluoride removal depending upon the end use of the material.
Standard petroleum refining industry practice removes organic fluorides and residual free hydrofluoric acid in the effluent streams of petroleum refining acid catalyst alkylation units by means of fixed bed decomposition and adsorption using high surface area activated alumina as the catalyst/adsorbent media. These fixed bed absorbers are referred to as Defluorinators. The term high surface area activated alumina refers to an aluminum oxide compound of the general formula AL2O.H2O having an extended surface area of above about 100 m2/g, preferably above about 150 m2/g. This degree of surface area imparts activity of the alumina towards a variety of chemical reactions.
In the present invention, the activated alumina catalyzes the decomposition of the combined fluoride to free hydrofluoric acid and olefin at elevated temperatures (350-475xc2x0 F., 175-250xc2x0 C.) as follows:
R2HCxe2x80x94CFR2xe2x86x92R2Cxe2x95x90CR2+HF
The free HF is removed from the process stream by subsequent reaction with the alumina to form aluminum trifluoride:
Al2O3+6HFxe2x86x922AlF3+3H2O
The present invention constitutes a new method for removing HF and related organic fluorides from fluid streams in which the fluoride species exist as impurities and, in particular, from hydrocarbon fluid streams containing up to 1000 ppm combined fluorides. The method of the invention consists of contacting the fluid stream with an adsorbent consisting essentially of activated alumina promoted with a compound selected from the oxides and phosphates of alkali metals and alkaline earth metals, and mixtures thereof. Thus, in essence, the method of this invention utilizes a base-promoted alumina adsorbent in place of the prior art non-promoted aluminas.
Bases utilized in this invention include alkaline and alkaline earth metal oxides and phosphates, and mixtures thereof. Particularly, the sodium, calcium, magnesium and potassium oxides and phosphates. When the base is an oxide, the activated alumina is promoted with Na2O and K2O, and preferably sodium oxide (Na2O). When the activated alumina is promoted with a phosphate, it may be selected from the group consisting of the phosphates of Li, Na, K, Be, Mg and Ca and, preferably, potassium phosphate. Cumulative promoter levels (oxide+phosphate) comprise between about 0.5 wt. % and about 25 wt. % of the activated alumina product. These bases are well known and have been used as promoters in other applications. However, their activity is generally based upon simple acid-base chemistry. Accordingly, the use of promoters with activated alumina in the context of the present invention was expected to provide additional HF loading capacity based upon that simple acid-base chemistry and the subsequent reaction of free HF with the promoter (together with the Al2O3 ), as shown below:
2HF+Na2Oxe2x86x922NaF+H2Oxe2x80x83xe2x80x83(Using: Na2O)
3HF+K3PO4xe2x86x923KF+H3PO4xe2x80x83xe2x80x83(Using: K3PO4)
Unexpectedly, however, trials under actual refinery conditions indicated performance benefits far beyond those anticipated, including: Increased fluoride-loading capacity on a per weight basis exceeding 40% as compared to current non-promoted products; significantly improved activity towards catalyzing the decomposition of organic fluorides; significantly improved resistance to carbon fouling and deposition, and significantly improved mechanical integrity of spent (fully reacted) material, thereby simplifying spent material handling and disposal.